Several applications require detection of one or several predefined tone(s) within received data. Such is the case, for instance, in telephone applications wherein signalling tones have to be detected embedded within voice signals or noise.
Correct detection and identification of the received tone is important in order to avoid false action being taken which would result in network mis-function.
The invention will be described more particularly as applied to so-called Multi-Frequency Receivers (MFR) wherein signalling tones combining two CCITT defined single frequency waveforms should be detected in a Touch Tone telephone network.
For instance, current telephone dialing systems are internationally defined by the CCITT, thus enabling common dialing throughout the world. Accordingly, depressing any telephone set dialing key generates a transmission over the telephone line, down to the PBX or CS, of a tone combining two single frequency waveforms. Each said waveform is taken within a predefined group of pure frequencies.
Conventional MFRs made to detect DTMFs (Dual Tone Multi Frequencies) in a digital environment are made of two filter banks, each bank covering a predefined bandwidth, by including one filter pre-tuned for each one of the CCITT defined frequencies. The received tones are detected by monitoring said pre-tuned filters for maximum energy and identifying the waveforms based on the corresponding filters.
Unfortunately, so-called transmission line "twisting" factors may alter differently the signals in the two bank's bandwidths, therefore impairing the tone detection quality.